Various devices (to be collectively referred to as gas insulated electrical equipment hereinafter) including gas insulated switchgears, gas circuit breakers, gas disconnecting switches, gas insulated transformers and gas insulated transmission lines that utilize sulfur hexafluoride gas (to be expressed as SF6 gas hereinafter) as insulation medium for high-voltage insulation are being employed in power transmission/distribution and transformation systems. SF6 gas is employed not only as insulation medium for high-voltage insulation but also as cooling medium for cooling various parts being heated by electricity by way of convection thereof and also as arc extinguishing medium for extinguishing arc discharges that can take place during switching operations of such devices as gas circuit breakers and gas disconnecting switches that involve electric current switching operations.
SF6 gas is a very stable inert gas. It is a gas that is non-toxic and non-inflammable and, at the same time, shows excellent electrical insulation performances and discharge extinguishing performances (to be referred to as arc extinguishing performance hereinafter). Thus, it has been and still is greatly contributing to realization of high performance and compact gas insulated devices for handling electric power to be used in power transmission/distribution and transmission equipment.
While SF6 gas is very suited for use in gas insulated electrical equipment, it is also known that SF6 gas has a strong global warming effect and hence the amount of use of SF6 gas has been desired to be reduced in recent years. The extent of global warming effect of a substance is generally indicated by a global warming potential, which is the ratio to the potential of CO2 gas that is equal to 1. It is known that the global warming potential of SF6 gas is as large as 23,900. For this reason, the use of gases other than SF6 that is a man made gas having a very large global warming potential has been proposed for use in gas insulated electrical equipment.
More specifically, the use of N2 gas, CO2 gas and mixture gases containing either of them as main components has been discussed and studied. N2 gas and CO2 gas are gases existing in nature and friendly to the environment. Since their global warming effects are very small and not greater than one 23,900th of that of SF6 gas, the influences of the use of insulation gases on global warming can significantly be suppressed by using N2 gas and CO2 gas instead of SF6 gas in gas insulated electrical equipment. While the insulation performance and the arc extinguishing performance of N2 gas and those of CO2 gas are inferior to those of SF6 gas, studies are being made to enhance the performances of those substitute gases by using higher filling pressure and improving the structures of devices using such gases. In other words, it is possible to provide gas insulated electrical equipment that generally shows excellent performances and is friendly to the environment and less global warming by using N2 gas and CO2 gas instead of SF6 gas.
While gas insulated electrical equipment is in operation, an arc (to be referred to as internal arc hereinafter) can take place in a hermetically sealed tank that contains the equipment. Once the internal arc occurs, excessive short-circuit currents flow there so that the pressure in the gas compartment in the hermetically sealed tank rapidly rises due to the large amount of heat generated by the short-circuit currents.
When the hermetically sealed tank is ruptured under such high pressure, it is very hazardous to the environment. For this reason, there are instances where a hermetically sealed tank is provided with a pressure release device for releasing the internal pressure of the hermetically sealed tank when the internal pressure abnormally rises. However, as pointed out above, SF6 gas that is being popularly employed at present is a substance that adversely affects the environment after discharged to the atmosphere and it is known that SF6 gas produces toxic decomposition products when SF6 gas is dissociated by the high temperature produced by the internal arc. Thus, hermetically sealed tanks and insulating spacers to be used therein are more often than not designed to show a considerably high mechanical strength in order to withstand the pressure produced by the internal arc in gas compartments and prevent gas from being released to the outside of hermetically sealed tanks even when the internal arc occurs in the inside of the tanks. Such efforts are desirable from the viewpoint of protecting the environment.